Sharp rise in heavy rain spells during the onset of northeast monsoon: study

There has been a sharp increase in rainfall during the onset phase of northeast monsoon in during 2011-2020 and the number of heavy rainfall incidences has also increased during the monsoon onset period, says a study that has been carried out by senior meteorologists of India Meteorological Department.
For the study, 16 coastal stations in the belt between south coastal Andhra Pradesh, north, central and south coastal Tamil Nadu were chosen. Some of the weather stations chosen were Nellore, Sulurpet, Chennai, Nungambakkam, Nagapattinam and Kanniyakumari .
The study noted that the daily rainfall had increased between 10 mm and 33 mm at the arrival of monsoon in October between 2011-2020. The daily rainfall during such period in previous decades usually was between 1 mm and 4 mm.
In its analysis on the frequency of heavy to extremely heavy rainfall in the region, it was revealed that there has been 429 heavy rainfall days for the 16 weather stations during the entire northeast monsoon in the decade.
Mr. Raj, one of the study’s authors, said the number of heavy rainfall occurrences was 91 days during the first week since the monsoon onset. The chances of heavy rainfall over coastal belt have increased by 19 times more during the monsoon set phase compared to pre-onset phase. However, such heavy rainfall incidences are rare after the monsoon withdrawal.

Noting that the onset and withdrawal dates are important features of the monsoon, the study said while the mean onset date was October 23, the average withdrawal date was December 31 in the decade. These were three and four days later respectively than the long period average dates.
The monsoon stayed for longer in south coastal Tamil Nadu till January 5.
The study had used superposed epoch technique to demonstrate the sharp increase and decrease of rainfall after the onset and withdrawal during the decade. It was based on the daily rainfall data between September and February obtained from National Data Centre, IMD, Pune.
Mr. Raj noted that the study was a sequel to earlier studies that aimed to generate historical data on monsoon onset and withdrawal dates for 140-year period since 1871. Places like Chennai have broken several heavy rainfall records in recent years and the city average annual rainfall has increased in recent decades.

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